This invention relates to a system and method of injecting an ammonia or urea reactant into a flue gas stream in a steam generating power plant that utilizes Selective Catalytic Reduction (SCR) to lower NO.sub.x emissions and more specifically to the injection of the reactant in such a manner that a more uniform mixing of the reactant with the flue gas stream is achieved as well as a more rapid mixing, thus increasing the efficiency of the catalytic reduction process; and further to a method of increasing the mass transfer limited reaction that takes place between the reactant/flue gas mixture and the catalytic material.
In recent years oxides of nitrogen, also known as NO.sub.x, have been implicated as one of the elements contributing to the generation of acid rain and smog. Now, due to very strict state and federal environmental regulations demanding that NO.sub.x emissions be maintained at acceptable levels, the reduction of NO.sub.x both during and after the combustion process is of critical importance and a major concern in the design and operation of modern power plants. Oxides of nitrogen are a byproduct of the combustion of hydrocarbon fuels, such as pulverized coal, gas or oil and are found in two main forms. If the nitrogen originates from the air in which the combustion process occurs, the NO.sub.x is referred to as "thermal NO.sub.x ". Thermal NO.sub.x forms when very stable molecular nitrogen, N.sub.2, is subjected to temperatures above about 2800.degree. F. causing it to break down into elemental nitrogen, N, which can then combine with elemental or molecular oxygen to form NO or NO.sub.2.
If the nitrogen originates as organically bound nitrogen within the fuel, the NO.sub.x is referred to as "fuel NO.sub.x ". The nitrogen content of coal, for instance, is comparatively small and, although only a fraction is ultimately converted to NO.sub.x, it is the primary source of the total NO.sub.x emissions from a coal-fired steam generating power plant.
One post-combustion process for the lowering of NO.sub.x emissions is that of Selective Catalytic Reduction (SCR). Selective Catalytic Reduction systems use a catalyst and a reactant such as ammonia gas, NH.sub.3, to dissociate NO.sub.x to molecular nitrogen, N.sub.2, and water vapor. The catalytic process using ammonia as a reactant is governed by the following chemical reactions: EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O (1) EQU 2NO.sub.2 +4NH.sub.3 +O2.fwdarw.3N.sub.2 +6H.sub.2 O (2)
Since NO.sub.x is approximately 95% NO in the flue gas stream, Equation 1 would dominate the process.
Urea is a promising SCR reactant that is coming into use because of some perceived advantages over ammonia. The catalytic process using urea as the reactant is governed by the following chemical equation: EQU CO(NH.sub.2).sub.2 +2NO 1/2O.sub.2 .fwdarw.2N.sub.2 +CO.sub.2 +2H.sub.2 O(3 )
A typical utility steam generating power plant utilizing Selective Catalytic Reduction as a NO.sub.x reduction technique comprises a furnace volume in fluid communication with a backpass volume. Combustion of hydrocarbon fuels occurs within the furnace volume creating hot flue gases that give up a portion of their energy to the working fluid of a thermodynamic steam cycle. The flue gases are then directed to and through the backpass volume wherein they give up additional energy to the working fluid. Upon exiting the backpass volume the flue gases are directed via a gas duct through a Selective Catalytic Reduction chamber and thence to either an air preheater or to flue gas cleaning systems and thence to the atmosphere via a stack.
The SCR reaction chamber typically includes several layers of solid catalytic material lying within the path of the flue gas stream. The most common types of catalytic material in use and the approximate temperature ranges of the flue gases over which they are effective as catalysts are: Titanium Oxide (270-400.degree. C.), Zeolite (300-430.degree. C.), Iron Oxide (380-430.degree. C.) and activated coal/coke (100-150.degree. C.). The type and amount of catalytic material for which an SCR system need be designed depends upon the flue gas volume, flue gas temperature, total NO.sub.x present in the flue gas, NO.sub.x reduction requirements, permissible ammonia slip, amount of SO.sub.x present in the flue gas stream and the uniformity of the concentration of the reactant in the flue gas stream as the mixture enters the SCR chamber. To help ensure that the temperature of the flue gas stream is within the aforesaid temperature ranges it is typical that a bypass duct is utilized that passes from the upper portion of the backpass volume to the gas duct such that still relatively hot flue gases are diverted from the backpass volume to a point in the gas duct upstream of the location of the injection of the reactant. This however may give rise to temperature gradients in the resultant flue gas stream. Furthermore, due to the geometry of the gas ductwork it is possible to acquire gradients in the mass flow rate of the flue gas across the flue gas stream upstream of the location of the injection of the reactant.
In a typical SCR system, at some point in the gas duct after the flue gas stream exits the backpass volume and after the bypass duct, yet still upstream of the SCR chamber, ammonia, in a gaseous or anhydrous form, or a urea/water solution is introduced into, and encouraged to mix with, the flue gas stream. The reactant/flue gas mixture then enters the SCR chamber where the catalytic reductions, shown in Equations 1, 2 or 3 take place.
Typical of prior art methods of introducing a reactant into a flue gas is seen in U.S. Pat. No. 4,297,319 which issued on Oct. 27, 1981 and is entitled "Apparatus For Removing Nitrogen Oxides From Flue Gas" and which teaches adding ammonia and hydrogen peroxide to the flue gas from an ammonia nozzle and a hydrogen peroxide nozzle. It is also seen in U.S. Pat. No. 4,309,386 which issued on Jan. 5, 1982 and is entitled "Filter House Having Catalytic Filter Bags For Simultaneously Removing NO.sub.x And Particulate Matter From A Gas Stream", that ammonia is introduced into a flue gas inlet conduit via an ammonia distribution grid. Furthermore, U.S. Pat. No. 5,296,206 which issued on Mar. 22, 1994 and is entitled "Using Flue Gas Energy To Vaporize Aqueous Reducing Agent For Reduction Of NO.sub.x In Flue Gas" teaches an injection grid of a known type and U.S. Pat. No. 5,326,536 which issued on Jul. 5, 1994 and is entitled "Apparatus For Injecting NO.sub.x Inhibiting Liquid Reagent Into The Flue Gas Of A Boiler In Response To A Sensed Temperature" teaches the use of a retractable spray nozzle inserted into the flue gas passageways in order to spray a liquid reagent. Still further, disclosed in U.S. Pat. No. 5,380,499, which issued on Jan. 10, 1995 and is entitled "Combined Heat Exchanger And Ammonia Injection Process", is a grid of injection pipes located in the flue gas ductwork and upstream of a SCR catalyst bed.
Also found in the prior art is the use of urea as a reactant. In particular U.S. Pat. No. 4,719,092 which issued on Jan. 12, 1988 and is entitled "Reduction Of Nitrogen-Based Pollutants Through The Use Of Urea Solutions Containing Oxygenated Hydrocarbon Solvents" teaches a process of injecting an aqueous solution of urea and an oxygenated hydrocarbon into an oxygen rich effluent from the combustion of a carbonaceous fuel. Furthermore, U.S. Pat. No. 5,139,754 which issued on Aug. 18, 1992 and is entitled "Catalytic/Non-Catalytic Combination Process For Nitrogen Oxides Reduction" teaches a process whereby a nitrogenous treatment agent is introduced into the effluent from the combustion of a carbonaceous fuel and wherein urea is much preferred as the nitrogenous treatment agent. Still further, U. S. Pat. No. 5,399,326 which issued on Mar. 21, 1995 and is entitled "Process For Noncatalytic NO.sub.x Abatement" teaches a process of injecting into a gas stream a liquid composition comprising urea, ammonia and water. Yet further, U. S. Pat. No. 5,478,542 which issued on Dec. 26, 1995 and is entitled "Process For Minimizing Pollutant Concentrations In Combustion Gases" teaches a process of injecting a NO.sub.x reducing mixture into a passage containing a combustion effluent and wherein the mixture contains a liquid component of a urea solution.
The design of an SCR system is dictated by such considerations as the concentration of NO.sub.x entering and leaving the SCR chamber, the flue gas temperature, the ammonia/NO.sub.x stoichiometric ratio, the flue gas volumetric flow rate and the available surface area of the catalytic material. To ensure optimized SCR operation, it is necessary that the distribution of the reactant across the flue gas stream be as uniform as possible, typically within +/-15% of an average value upon entering the SCR chamber. However, a more uniform mixing helps ensure that the catalytic reduction occurs more evenly over all of the surface of the catalytic material and a more efficient use of the catalytic material is realized. As a consequence, for a fixed cross sectional area of catalytic material, the amount of catalytic material for which an SCR system need be designed may be reduced; thus reducing pressure losses in the power plant and thereby improving plant efficiency.
It is desirable that the aforesaid mixing be completed before the mixture enters the SCR chamber in order to optimize completion of the catalytic reduction therein. However, the reactant mixing requires a certain amount of time to complete, and as a consequence, at fixed flue gas velocities, also requires that a certain length of ductwork be made available from the point of injection of the reactant into the flue gas stream to the entrance to the SCR chamber. In a power plant that is being retrofitted with an SCR system there may in fact be very limited ductwork available in which to allow time for the necessary mixing and in the design of a new power plant there may be limited area available for plant layout. Thus, by increasing the rapidity with which the reactant mixes with the flue gas stream, an opportunity is made available for a reduction in the length of ductwork necessary from the point of injection of the reactant to the entrance to the SCR chamber.
The maximum allowed additional pressure drop in a typical retrofit SCR application is about 4" WG. Of that pressure drop about 3" WG may be directly attributable to the presence of the catalytic material. However, in many retrofit applications, the SCR chamber is typically forced in between the existing economizer outlet and the air heater inlet with little opportunity for designing aerodynamically sound connecting ductwork. To meet strict inlet flue gas and reactant flow profile requirements at the initial surface of the catalytic material, flow controls of some type are required. Typically a series of controls, employed to ensure uniform mixing of the reactant across the flue gas stream, are located after the point at which the reactant is injected into the flue gas stream. This is followed by a series of controls, located just prior to the entrance to the SCR chamber, to enforce a more uniform mass flow rate of flue gas across the surface of the catalyst. However, these controls impose additional pressure losses upon the system. Therefore, it is desirable that the means by which the reactant is introduced into the flue gas stream cause a minimal increase in the total pressure coefficient, K, of the power plant. The total pressure coefficient, K, is the ratio of the static head to the dynamic head of a flowing fluid and can be thought of as a measure of the resistance offered to the flow of the flue gas stream.
Thus, current methods of introducing a reactant into the flue gas stream, while sometimes capable of improving upon the uniformity of the distribution of reactant across the flue gas stream, still leave room for improvement in the uniformity distribution and for allowing mixing to occur in a sufficiently rapid manner.